"Using the example above of a 250-watt STC rated panel, if you multiply the 250 watts the panel produces by the number of hours of full sun you get in a day, you’ll get the amount of kwh that panel produces per day. Multiply by 30 days and you’ll get mothly kWh output for the panel.
The average roof in the United States gets about 4 hours of usable sun per day. We know the sun shines more than 4 hours, but “full sun” is a measurement that combines all the parts of the day when the sun is lower in the sky into one number.
Using 4 hours of full sun, gives you this equation: 250 watts x 4 hours. That’s 1 kWh (1,000 watts) in a day per 250-watt panel.
If you multiply 1kWh per panel by 30 days in a month, you’ll find that each 250 watt rated panel will produce about 30 kWh in an average month."
This makes sense to me, but when I take our soon to be installed 57 305 Watt panels (17.385kW according to my calculations), it doesn't.
With the array size being 17.385kW (69.54kWh generated in an average day, according to the above, if I am not mistaken) to only meet our daily usage of 41.73kwH (1,252kWh is our average monthly usage) out of the array per our solar company, it is only on ever up to 60% efficient.
Does this sound right to others also?